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Sep 2016
AN57U Rev A
Apex Inkjet Printhead Driver Power Dissipation
AN57
INTRODUCTION
Printing industries are rapidly converting from conventional techniques, like offset lithography and roto‐
gravure, to digital technology like inkjet. The printheads of many industrial inkjet printers use piezoelectric
technology. In the printheads of so called ‘drop‐on‐demand’ (DoD) inkjet printers the piezoelectric nozzles
produce ink drops when driven by electric pulses. Since the nozzles behave like electric capacitors and the
amount of nozzles in a printhead being fired varies continuously, the amplifier driving the printhead ‘sees’ a
very dynamic, capacitive load. Apex Microtechnology’s power operational amplifiers are very well suited to
drive DoD inkjet printheads as they meet the power requirements of and can be tuned to this specific applica‐
tion by external components. This application note describes a way to derive the internal power dissipation
of an Apex power op amp driving a DoD inkjet printhead.
THE AMPLIFIER AND THE FIRE PULSE
DoD piezoelectric printheads usually contain a large, even number of nozzles, often a power of 2, that
can be in a single or in multiple rows. The head also contains digital circuitry through which each individual
nozzle can be set to eject a drop at the next fire pulse or to stay inactive. The active nozzles are all in parallel
when the driver amplifier produces a fire pulse to make them all eject drops simultaneously. Single nozzle
capacitance, depending on manufacturer, can vary wildly, but usually is in the range of 150pF to 2nF.
A typical amplifier circuit and fire pulse would look like figure 1.
Figure 1: Typical Amplifier Circuit and Fire Pulse
HEADROOM AND LEGROOM
Since power operational amplifiers exhibit a voltage drop across the conducting output transistor, which
depends on the output current, they cannot swing rail to rail. In order to achieve a certain output voltage
swing, the amplifier needs to be powered off of rails providing enough headroom at the topside and legroom
at the bottom side. The supply rails of the power operational amplifier should be chosen such as to accom‐
modate the largest fire pulses.
Legroom
Headroom
T
r
Vpulse
Vmid
Tf
Tpulse
Rf
Ri
Cc
Piezo
Nozzle
(s)
-Vs
+Vs
AN57
2 AN57U Rev A
SLEW RATE
Slew Rate is the rate of change of the output voltage of the amplifier per unit of time. In figure 1:
with V
pulse
being the amplitude of the fire pulse and T
r
and T
f
being the times it takes the amplifier to pro‐
duce the rising and falling edges of the fire pulse respectively. For the purpose of this application note it is
assumed T
r
=T
f
, but this can be different in practice.
V
DRIVE,MIN
AND V
DRIVE,MAX
V
drive,min
and V
drive,max
are the voltage extremes of the fire pulse, with V
drive,min
being the bottom and
V
drive,max
being the top value. For the purpose of this application note it is assumed that subsequent pulses
have the same amplitude, but in practice, when grey scale printing is being achieved through a technique
called multi‐pulsing the printhead, individual pulses within pulse trains can have different amplitudes, thus
manipulating the drop size.
V
MID
V
mid
is the voltage of the fire pulse, halfway between its minimum and maximum drive voltages. See remark
about multi‐pulsing under V
drive,min
and V
drive,max
. If subsequent pulses have different amplitudes, their mid‐
points are different, too.
T
PULSE
T
pulse
is the fire pulse’s period.
CALCULATION OF INTERNAL POWER DISSIPATION
Total power dissipation in the power operational amplifier consists of two components: 1) Quiescent
Power Dissipation and 2) Output (Stage) Power Dissipation.
1. Quiescent power dissipation is caused by the quiescent current draw of the power op amp. This current is
used internally to bias the various stages of the amplifier. It also flows when the amplifier is idling. The
quiescent power dissipation can be calculated as:
with I
Q
being the quiescent current and V
SS
the total voltage across the amplifier or +V
S
‐‐V
S
. This means that
even if the amplifier is doing nothing, it may already need to be cooled down by mounting it on a heatsink!
2. Output (stage) power dissipation is caused by the conducting output stage transistor, dropping a certain
voltage from the supply rail to produce the required output voltage, and the output current that flows
through this transistor.
The output current is:
The voltage across the conducting output stage transistor varies during the rising and falling edges of the
fire pulse signal. Since the current is (assumed to be) constant, it can be assumed constant as well at the mid‐
point between the voltage extremes of the pulse.
SR
V
pulse
T
r
orT
f
---------------------
=
1
T
pulse
--------------
f
pulse
Hz=
P
Q
I
Q
V
SS
=
I
o
C
dV
dt
-------
A��=